manager: comprehensively mark manager_dispatch_user_lookup_fd() as static

The prototype was static, but the implementation was not. Make both
static, this is otherwise too confusing. (This doesn't actually change
anything, since the prototype decides about this anyway, but it makes
things easier to read.)

src/core/manager.c

@@ -4755,7 +4755,7 @@ static void manager_vacuum(Manager *m) {
         exec_shared_runtime_vacuum(m);
 }
 
-int manager_dispatch_user_lookup_fd(sd_event_source *source, int fd, uint32_t revents, void *userdata) {
+static int manager_dispatch_user_lookup_fd(sd_event_source *source, int fd, uint32_t revents, void *userdata) {
         struct buffer {
                 uid_t uid;
                 gid_t gid;